The fourth generation, 4G, wireless access within the 3rd generation partnership project, 3GPP, long-term evolution, LTE, is based on orthogonal frequency-division multiplexing, OFDM, in downlink and discrete Fourier transform, DFT, spread OFDM, also known as single carrier frequency division multiple access, SC-FDMA, in uplink. Here, with reference to FIG. 2, the uplink consists of the physical channels physical uplink shared channel, PUSCH, physical uplink control channel, PUCCH, and physical random access channel, PRACH, as well as of physical signals referred to as the demodulation reference signal, DMRS, and the sounding reference signal, SRS. According to the 3GPP specification, see 3GPP TS 36.211 V11.3.0, the PUSCH, PUCCH, DMRS, and SRS all use an inverse fast Fourier transform, IFFT, of size 2048 in the transmitter, with a sampling rate of 30.72 MHz, see FIG. 2. The same size of 2048 can be used for the fast Fourier transform, FFT, in the receiver. Dedicated hardware is commonly used for these FFTs. With another sampling rate than 30.72 MHz, the IFFT and FFT size will change accordingly.
The physical random-access channel, i.e., the PRACH, is used for initial access for a wireless device into the radio access network and also for timing offset estimation, i.e., estimation of timing offset between wireless device transmissions and reception at, e.g., a base station or other receiver in the radio access network. A description of this random access procedure is given in 3GPP TS 36.213, V11.3.0. The PRACH formats, as specified for LTE, see, e.g., 3GPP TS 36.211, V11.3.0, comprise five different formats where a PRACH preamble consists of one or two sequences, each of length 24 576 samples. The preambles of some formats have a cyclic prefix, CP, of length between 3 168 and 21 024 samples, as shown in FIG. 2.
Several methods have been proposed for how to detect the PRACH preambles transmitted by the wireless device, see e.g., S. Sesia. I. Toufik. M Baker “LTE, The UMTS Long Term Evolution, From Theory to Practice”, Second Edition, John Wiley & Sons Ltd., 2011.
Many of the proposed preamble detection methods have in common that they require a large FFT, often significantly larger than the FFT used to detect OFDM symbols transmitted, e.g., on the PUSCH, as illustrated in FIG. 2. This large FFT drives complexity and power consumption in many systems, and potentially also increases the need for cooling of the receiver.
Implementing methods that require a large FFT can be especially burdensome in emerging fifth generation, 5G, technologies, where the use of very many antenna elements is foreseen. This is because the large FFT must typically be determined for each separate antenna, or subset of antennas, such that different users and channels in different sub-bands of the received signal can be extracted before further signal processing.
Furthermore, the PRACH preamble as specified in LTE, see table 5.7.1-1, section 5.7.1 in 3GPP TS 36.211, V11.3.0, covers a time interval which is much longer than the length of OFDM symbols used for other transmissions such as user data symbols. Current PRACH preamble receivers are thus designed under the assumption that propagation conditions do not vary significantly during the length of the preamble. This may be problematic, since assumptions, or constraints, are placed on the communication system. These constraints include expectations on low wireless device speed, i.e., Doppler spread, low frequency errors and low Doppler shifts, and also low phase noise in transmitters and receivers.
Thus, there is a need for an improved random access signaling technique, i.e., a preamble sequence transmitter and receiver, which does not place or otherwise imply the above mentioned constraints on the communication system, and which allows for both reliable and efficient random access detection as well as accurate timing offset estimation of a received random access signal, in small cells as well as in large cells.